The performance of passive optical components depends on the propagation of the light through a more of less complex set of optical paths of the device. A measurement technique able to provide the characterising parameters, for example phase and amplitude of the propagation of the light through the device is a very powerful tool for the analysis, development and improvement of passive optical components.
Arrayed waveguide gratings (AWG) are key components for high-speed large-capacity dense wavelength division multiplexing (DWDM) photonic networks. They are good candidates for dense channel de/multiplexers and building blocks of devices with other functionalism e.g.: dispersion compensators, optical add and drop multiplexers, gain equalisers, etc. Due to the increasing of the transmission rate per channel, the specification regarding the slope of the group delay (GD) and the differential group delay (DGD) are becoming narrower, so that new high precision characterisation methods and equipment are required. From other side, the amplitude and phase distributions of the light propagating through each kth arrayed path, (αk, φk), is a very useful information to investigate and improve the fabrication process of the AWGs.
A solution based on Fourier Spectroscopy Technique, namely the so called Optical Low Coherence (OLC) method, is known from IEEE Journal of Lightwave Technology, 1996, 14, pp. 1677–1689; TAKADA, K. YAMADA H., INOUE, Y. “Optical Low Coherence Method for Characterizing Silicia-Based Arrayed-Waveguide Grating Multiplexers”. The OLC method is in particular useful to characterise and improve AWG's, because it provides the amplitude and phase distributions (ηk, φk) of the light propagating through each arrayed waveguide of the array, k. By applying the Fourier Transform Spectroscopy (FTS), using a low coherence interferometer, an interferogram with a series of isolated fringes is provided, from which the amplitude and phase (αk, φk) of each arrayed-wave guide can be derived using the OLC method.
However, the solution is from basic principles and has a limited application, because it is not possible to analyze optical devices with a small or large optical path differences. The applicability of the OLC method is limited on the one hand by the coherence length of the available light sources, because it must be shorter than the delay increment (ΔL) of the AWG. In other words: In order to get the isolated fringes, it is necessary to apply a broadband light source with a coherence length shorter than the ΔL of the AWG. Thus, the applicability of this method to AWG designs with narrow ΔL is therefore limited by the bandwidth of the available light sources. On the other hand, it is limited by the maximum retardation of the interferometer, which is required to be more that half the maximum optical path difference among the arrayed waveguides. This is impossible for infinite impulse response filters.